Clashes between rebels and government forces have wrecked food markets and forced people to abandon their livestock and land, the aid experts added.
"We are losing time. Farmers should be planting their crops right now," Valerie Amos, the UN's aid chief, told a donors' conference in Oslo.
"If they don't, and if livestock herders are not able to migrate to grazing areas, people will run out of food."
Violence erupted in the oil-producing country in December following a long power struggle between President Salva Kiir and his sacked deputy Riek Machar.
UN officials in Oslo said the needed $1.8 billion in aid, up from their previous figure of $1.3 billion.
Yesterday, donors including the United States, Britain and Norway, agreed to give more than $600 million, on top of the $536 million already pledged.